1. Field of the Invention
The present invention relates to a method for fabricating a bump forming plate member and a method for forming bumps such as solder bumps arranged at a predetermined pattern for mounting an electronic component such as a semiconductor chip to a printed circuit board.
2. Description of the Related Art
With recent requirement to make electronic apparatuses smaller and lighter, electronic components such as ICs and LSIs are made by including highly integrated elements on semiconductor chips, and the number of input and output terminals has increased to several hundreds, for example. In a known method for mounting the electronic components having such a large number of terminals to printed circuit boards, bumps such as solder bumps are formed on the surface of the electronic components in advance, and the bumps are fused to electrode pads on the printed circuit boards.
Conventionally, as means for forming bumps on the surface of the electronic components in advance, a plating method, a vapor deposition method, and a transferring method are known. FIG. 23 in the attached drawings shows a conventional bump transferring method, in which a bump forming plate member 1 is used on an electronic component 2 to transfer solder. The electronic component 2 has a plurality of electrode pads 3 arranged at a predetermined pattern, and the bump forming plate member 1 has through holes 4 arranged at a pattern corresponding to that of the electrode pads 3.
In the transfer process, the bump forming plate member 1 is placed on the electronic component 2 with the through holes 4 aligned with the electrode pads 3, and the solder paste 6 is filled in the through holes 4 of the bump forming plate member 1 by a squeegee 5. The bump forming plate member 1 is then lifted relative to the electronic component 2, so that the solder paste 6 is released from the through holes 4 of the bump forming plate member 1 and transferred to the electronic component 2. The electronic component 2 is then placed on a printed circuit board and heated, to mount the electronic component 2 onto the printed circuit board.
The problem of the transferring method of FIG. 23 is that when the bump forming plate member 1 is lifted relative to the electronic component 2, the solder paste 6 sometimes sticks to the wall surface of the through holes 4 of the bump forming plate member 1 and is not transferred to the electronic component 2. As the pitch of the pin arrangement of the recent electronic component 2 becomes smaller, the cross-sectional area of the through holes 4 of the bump forming plate member 1 becomes smaller, so the solder paste 6 is not released from the through holes 4 of the bump forming plate member 1.
FIG. 22 in the attached drawings shows a bump transferring method described in Japanese Unexamined Patent Publication (Kokai) No. 4-263433. A bump forming plate member 1 has grooves 4a which are arranged at a pattern corresponding to that of the through holes 4 of FIG. 23. However, the grooves 4a are not through holes. In the case of FIG. 22 too, the solder paste 6 is filled in the grooves 4a of the bump forming plate member 1 by a squeegee 5. The bump forming plate member 1 is then placed on an electronic component 2, and the electronic component 2 and the bump forming plate member 1 are heated so that solder component in the solder paste 6 is rounded to form solder balls 6a which partly project from the surface of the bump forming plate member 1. When the electronic component 2 is lifted relative to the bump forming plate member 1 and the solder balls 6a are transferred from the bump forming plate member 1 to the electronic component 2.
In this transferring method, there is no problem as to the fact that the solder paste 6 sometimes sticks to the wall surface of the through holes 4 of the bump forming plate member 1. However, since the grooves 4a of the bump forming plate member 1 have a constant cross section from the top to the bottom thereof, the solder balls 6a formed by heating the solder paste 6 contact the side surface of the grooves 4a, there is a case in which the solder balls 6a falls off the grooves 4a when the electronic component 2 is lifted relative to the bump forming plate member 1.
Therefore, it is necessary to form the grooves 4a such that the grooves 4a have a relatively large cross section and a relatively small depth so that the solder balls 6a do not contact the side surface of the grooves 4a. However, this necessitates an increase in the pitch between two adjacent grooves 4a and cannot satisfy the narrow pitch requirements. In addition, when the bump forming plate member 1 is made from a material such as a stainless steel plate or a glass plate, it is possible to form grooves 4a in the surface of the stainless steel plate or the glass plate, by boring or drilling, or by etching. However, it is difficult to form the grooves 4a in such a manner that the grooves 4a have an identical shape and are arranged at a constant pitch.
FIG. 21 in the attached drawings shows a further bump transferring method. A bump forming plate member 1 has grooves 4b which are arranged at a pattern corresponding to that of the through holes 4 of FIG. 22. In this case too, the solder paste 6 is filled in the grooves 4b of the bump forming plate member 1 by a squeegee 5 (not shown), solder balls 6a are formed by heating the solder paste 6, and the solder balls 6a are transferred from the bump forming plate member 1 to the electronic component 2 (not shown).
The bump forming plate member 1 shown in FIG. 21 is made from a crystalline plate such as silicon. The grooves 4b are formed by subjecting the crystalline plate to anisotropic etching. In this arrangement, it is possible to obtain the bump forming plate member 1 in such a manner that the grooves 4b have an identical shape and arranged at a constant pitch, and it is possible to form bumps on the electronic component 2 arranged at a constant narrow pitch, using the bump forming plate member 1.
The grooves 4b of the bump forming plate member 1 of FIG. 21 have a triangular shape in cross section with the wide top and the narrow bottom, so the solder balls 6a can be easily released off the grooves 4b without sticking thereto when the solder balls 6a are transferred from the bump forming plate member 1 to the electronic component 2.
When the grooves 4b are formed by subjecting the crystalline plate to anisotropic etching, it is possible to form the grooves 4b having an identical shape and arranged at a constant pitch, but the grooves 4b have a final shape in which the inner surface of the grooves 4b are inclined surfaces and the depth D of the grooves 4b is decided depending on the size L of the opening of the mask 7 (the lip of the groove 4b). When the pitch of the pin arrangement of the electronic component 2 becomes smaller, the size L of the lip of the groove 4b becomes smaller and the depth D of the grooves 4b becomes smaller. As a result, the size of the solder balls 6a thus formed becomes smaller.
In addition, the bump forming plate member 1 having the grooves 4b is made of a very thin crystalline plate, so it is relatively fragile. However, in the prior art, it is not possible to form small grooves 4b arranged at a small pitch if the bump forming plate member 1 is made of a metal plate. However, it is desired that the bump forming plate member 1 is made from metal and has the grooves 4b arranged at a small pitch.